1. Field of the Invention
The present invention relates to a method for treating carbon fibers and, in particular, to a method for producing chemically active centers on surfaces of carbon fibers.
2. Brief Description of the Prior Art
Prior art composite materials that are reinforced with carbon fibers do not provide the desired mechanical properties because a sufficiently strong interface between the fibers and the matrix material is lacking. Known methods for improving the interface strengths concentrate on modifying the fiber surfaces through application of coatings, or through the treatment of the fibers with oxidative solutions that include an active oxidant. However, the beneficial effects of the oxidative treatments are not certain. It is believed that oxidation improves the interface strength by increasing the fiber surface area; by depositing chemical functionalities on the fiber surface, which increase the strength of the individual chemical bonds with the matrix; and by removing defects from the fiber surface. However, the mechanisms by which oxidative treatments improve interfaces and why the improvements are not as great as desired have been matters of controversy.
Moreover, while it is known that exposed surfaces of carbon fibers are basal plane graphite, the structural and chemical properties associated with the basal plane nature of the fiber surfaces have not been utilized for producing active centers on the surfaces of the carbon fibers.
According to J. B. Donnet, Carbon 20, 267 (1982), all oxidative solutions fall either into the class of graphitic oxide formers (i.e., solutions of the type that produce graphitic oxide), which preferentially attack the graphitic basal plane surface, or into the class of CO.sub.2 formers (i.e., solutions of the type that produce CO.sub.2), which preferentially attack the edges of the basal planes. Typically, the oxidative solutions that have been used for surface treatments are members of the class of CO.sub.2 formers. However, such solutions have no effect on the basal plane areas of the fibers where bonding sites are absent. The principle mechanism by which the conventional fiber treatment reagent improves interface strength is by decreasing the size of the inert basal plane domains through erosion of the domain edges. This is a highly inefficient mechanism whose benefits are strongly limited by the degradation that accompanies this type of surface structure modification. Consequently, the expected interface strengths have not been achievable by this type of oxidation treatment. Also, since the number of native edge sites decreases with fiber modulus, the beneficial effects of conventional oxidation treatments diminish with fiber modulus. In the very high modulus range, where the interface problem is most severe, conventional oxidative treatments provide little or no benefit.
Although the class of oxidative solutions that form graphitic oxide on the basal plane preferentially attacks the basal plane surface, these solutions are infrequently used in the treatment of carbon fibers. The present inventor has surprisingly found that the chemical disruption of basal plane areas by graphitic oxide formers produces chemically active sites precisely where they are needed on carbon fiber surfaces.
In the few cases where carbon fiber treatments with these reagents have been reported the oxidative solutions were not used in a way that was appropriate to the controlled activating of the fiber surface. The oxidation reaction was allowed to go to the point of graphitic oxide formation and attention was focussed on the nature of the chemical functionalities that the reaction produced on the surface. Fabrication of composites with such fibers produced improved interface properties, but the presence of the graphitic oxide leaves a great deal of uncertainty as to the mechanism of the improvement. In no case has an oxidative solution been used to chemically activate the inert basal plane areas of carbon fiber surfaces without the formation of significant quantities of graphitic oxide. Also graphitic oxide formers have not been previously used with CO.sub.2 formers in a manner so that the chemically active sites formed are maximized.